Ozone. A gas that can be found in two places. Near the ground (the troposphere), it is a major part of smog. Higher in the air (the stratosphere), it helps block radiation from the sun.

Ozone is not created directly, but is formed when nitrogen oxides and volatile organic compounds mix in sunlight. That is why ozone is mostly found in the summer. Nitrogen oxides come from burning gasoline, coal, or other fossil fuels. There are many types of volatile organic compounds, and they come from sources ranging from factories to trees.

Ozone near the ground can cause a number of health problems. Ozone can lead to more frequent asthma attacks in people who have asthma and can cause sore throats, coughs, and breathing difficulty. It may even lead to premature death. Ozone can also hurt plants and crops.

Carbon monoxide. A gas that comes from the burning of fossil fuels, mostly in cars. It cannot be seen or smelled.

Carbon monoxide is released when engines burn fossil fuels. Emissions are higher when engines are not tuned properly, and when fuel is not completely burned. Cars emit a lot of the carbon monoxide found outdoors. Furnaces and heaters in the home can emit high concentrations of carbon monoxide, too, if they are not properly maintained.

Carbon monoxide makes it hard for body parts to get the oxygen they need to run correctly. Exposure to carbon monoxide makes people feel dizzy and tired, and gives them headaches. Elderly people with heart disease are hospitalized more often when they are exposed to higher amounts of carbon monoxide.

Nitrogen dioxide. A reddish-brown gas that comes from the burning of fossil fuels. It has a strong smell at high levels.

Nitrogen dioxide mostly comes from power plants and cars. Nitrogen dioxide is formed in two wayswhen nitrogen in the fuel is burned, or when nitrogen in the air reacts with oxygen at very high temperatures. Nitrogen dioxide can also react in the atmosphere to form ozone, acid rain, and particles.

High levels of nitrogen dioxide exposure can give people coughs and can make them feel short of breath. People who are exposed to nitrogen dioxide for a long time have a higher chance of getting respiratory infections. Acid rain can hurt plants and animals, and can make lakes dangerous to swim in or fish in.

Particulate matter. Solid or liquid matter that is suspended in the air. To remain in the air, particles are usually less than 0.1 mm wide, and can be as small as 0.00005 mm.

Particulate matter can be divided into two typescoarse particles and fine particles. Coarse particles are bigger than 0.002 mm, and are formed from sources like road dust, sea spray, and construction. Fine particles are smaller than 0.002 mm, and are formed when fuel is burned in automobiles and power plants.

Particulate matter that is small enough can enter the lungs and cause health problems. Some of these problems include: more frequent asthma attacks, more trips to the hospital for respiratory problems, and premature death. Particulate matter can also make clothes and other materials dirty.

Sulfur dioxide. A corrosive gas that cannot be seen or smelled at low levels but can have a rotten egg smell at high levels.

Sulfur dioxide mostly comes from the burning of coal or oil in power plants. It also comes from factories that make chemicals, paper, or fuel. Like nitrogen dioxide, sulfur dioxide also reacts in the atmosphere to form acid rain and particles.

Sulfur dioxide exposure can affect people who have asthma or emphysema by making it more difficult for them to breathe. It can also irritate people's eyes, noses, and throats. Sulfur dioxide can harm trees and crops, damage buildings, and make it harder for people to see long distances.

Lead. A blue-gray metal that is very toxic and is found in a number of forms and locations.

Outside, lead comes from cars in areas where unleaded gasoline is not used. Lead can also come from power plants and other industrial sources. Inside, lead paint is an important source of lead, especially in houses where paint is peeling. Lead in old pipes can also be a source of lead in drinking water.

High amounts of lead can be dangerous for small children, and can lead to lower IQs and kidney problems. For adults, exposure to lead can increase the chance of having heart attacks or strokes.

Toxic air pollutants. A large number of chemicals that are known or suspected to cause cancer. Some important pollutants in this category include arsenic, asbestos, benzene, and dioxin.

Each toxic air pollutant comes from a slightly different source, but many are created in chemical plants or are emitted when fossil fuels are burned. Some toxic air pollutants, like asbestos and formaldehyde, can be found in building materials and can lead to indoor air problems. Many toxic air pollutants can also enter the food and water supply, and people can be exposed when they eat or drink.

Toxic air pollutants can cause cancer. Some toxic air pollutants can also cause birth defects. Other effects depend on the pollutant, but can include skin and eye irritation and breathing problems.

Stratospheric ozone depleters. Chemicals that can destroy the ozone in the stratosphere. These chemicals include chlorofluorocarbons (CFCs), halons, and other compounds that include chlorine or bromine.

CFCs are used in air conditioners and refrigerators, since they work well as coolants. They can also be found in aerosol cans and fire extinguishers. Other stratospheric ozone depleters are used as solvents in industry.

If the ozone in the stratosphere is destroyed, people are exposed to more radiation from the sun (ultraviolet radiation). This can lead to skin cancer and eye problems. Higher ultraviolet radiation can also harm plants and animals.

Greenhouse gases. Gases that stay in the air for a long time, and warm up the planet by trapping sunlight. This is called the greenhouse effect because the gases act like the glass in a greenhouse. Some of the important greenhouse gases are carbon dioxide, methane, and nitrous oxide.

Carbon dioxide is the most important greenhouse gas, and it comes from the burning of fossil fuels in cars, power plants, houses, and industry. Methane is released during the processing of fossil fuels, and also comes from natural sources like cows and rice paddies. Nitrous oxide comes from industrial sources and decaying plants.

The greenhouse effect can lead to changes in the climate of the planet. Some of these changes might include more temperature extremes, higher sea levels, changes in forest composition, and damage to land near the coast. Human health might be affected by diseases that are related to temperature, or by damage to land and water.

Source: Jonathan Levy, Harvard School of Public Health. Based on information provided by the Environmental Protection Agency.